1. Field of the Invention
This invention relates generally to batteries made from two or more battery cells connected in parallel, and more particularly to systems and methods for managing such batteries.
2. Description of the Related Art
With the growing requirements of high-energy battery-operated applications, the demand of multi-cell rechargeable battery packs (or simply “batteries”) has been increasing drastically. Multiple cells are needed to serve the high capacity/energy requirements of certain battery applications, such as a laptop computer. Within a multi-cell battery pack, there may be multiple cells connected in series, as well as multiple cells connected in parallel. For example, a battery with four 3.6-volt cells connected in series may give a nominal voltage of 14.4V, with a capacity of 1000 milli-Amp-hours (mAh). ‘N’ groups of four series-connected 3.6-volt cells—each of which forms a “series stack of cells”—may then be connected in parallel to increase the battery's capacity from 1000 mAh to N*1000 mAh. At this time, popular multi-cell rechargeable batteries used in handheld appliances, computers, power tools, etc., are rather expensive and range from US$30 to US$300, depending on the number of cells and their respective capacities in the pack.
In use, rechargeable batteries are subject to many ‘charge/discharge cycles’, each of which consists of a battery charging operation which is intended to charge all cells up to their full capacity, followed by a period during which the cells lose their charge, through active use and/or inevitable leakage. One of the key challenges in charging/discharging multi-cell batteries is related to the non-uniformity of battery cells within the pack, due to, for example, manufacturing tolerances. For example, some ‘weak’ cells have a lower capacity than other cells, such that when fully charged, the weak capacity cell will provide less charge during operation than the other cells. A weak battery cell tends to limit the overall capacity of the entire battery. One way in which this problem is addressed is to use a special manufacturing process which involves binning and grouping cells based on their capacity properties, and then forming a battery pack using cells from the same bin. However, this extra step increases manufacturing cost. Moreover, mismatch between the cells increases after multiple charge/discharge cycles, which reduces the benefit of binning at the factory.
In addition, a battery pack that includes one or more series stacks of battery cells may no longer function if any given cell in a stack is severely degraded. In other words, the battery packs life time may be significantly degraded due to one single damaged cell. Even if cells are connected in parallel in a battery pack, different cells may lose different amounts of their capacity with each charge/discharge cycle. The entire pack may be unable to store its required charge even if only a few cells or a single cell lose a significant amount of capacity. Also, if only one cell fails by forming a short circuit, the entire pack is destroyed. Even though different cells may be able to withstand different numbers of charge/discharge cycles, they all must experience the same number of charge discharge cycles if they are tied together in parallel.